1. Field of the Invention
The present invention relates to a power line communication apparatus to which a band window for band reservation and a communication band for data communication are alternately assigned at predetermined intervals, as well as to a method for operating that power line communication apparatus.
2. Description of the Related Art
There are a number of types of communication apparatuses that utilize the power line for communication purposes. Such communication apparatuses may illustratively adopt, as their modulation system, ASK (amplitude shift keying), FSK (frequency shift keying), or PSK (phase shift keying). As their transmission system, the apparatuses may typically select the single carrier scheme, OFDM (orthogonal frequency division multiplexing), or spread spectrum communication system.
The communication apparatuses of such different systems are known to be incapable of communicating with one another. For example, operating on the same OFDM principle, the apparatuses are unable to communicate with one other if they differ in terms of any one of such system specifications as occupied bandwidth, subcarrier cycles, subcarrier modulation, synchronizing signal, encoding method, access control, and data format. In the description that follows, the term “different systems” will refer to systems with differences therebetween in terms of their communication specifications.
If apparatuses of different systems incommunicable with each other are installed in the same home, their outgoing signals can collide with each other, making communications difficult. In the worst case, all apparatuses could become incapable of communicating with one another. As power line communication apparatuses come into general use from now on, it will become important to devise means and arrangements to minimize or forestall interference between the communication apparatuses of different systems so that they can coexist in a delimited environment.
In order to ensure the coexistence of different wireless communication systems, there have been proposed techniques for controlling the timings of communication between such systems using a signal (beacon signal) that can be exchanged therebetween. One such technique is disclosed illustratively in Japanese Patent Laid-open No. 2003-258812, referred as Patent Document 1.
In a common wireless LAN setup, the beacon signal is typically used as follows: each master station periodically sends out a wireless beacon signal (a kind of OFDM-modulated control packet). A slave station receives the beacon signal on all wireless channels, and selects an optimal master station based on the received beacon signal. The slave station proceeds to request authentication from the selected master station. In turn, the master station determines whether or not to authenticate the slave station requesting connection to the network of its own. If the requesting slave station is successfully authenticated, the master station exchanges management signals with the authenticated station and thereafter treats the slave station as a legitimate wireless LAN terminal.
That is, the beacon signal is a sort of intra-system control signal generated by the master station as a wireless identification signal that identifies the network managed by the master station in question and the devices connected to that network (the network and the connected devices will be generically referred to as the system hereunder). The invention disclosed by Patent Document 1 involves furnishing the beacon signal, commonly defined between systems, with detailed control information such as transmission parameters so as to secure the coexistence of different systems.
In the field of power line communication, there have been proposals to define a CDCF (commonly distributed coordination function) signal as a coexistence control signal (simply called the coexistence signal) recognizable by modems (e.g., PLC modems) or other suitable communication apparatuses of different systems to ensure their coexistence, the CDCF signal being used as the basis for working out the system specifications for band reservation. One such proposal is found in Non-Patent Document 1 titled “Overview of CEPCA and Approach to PLC Standardization,” Jul. 19, 2006 (presented by Mark Eyer, Sony), pp. 26-28.
In power line communication, the use of existing power lines has made system standardization a complex and difficult exercise. It is preferred that the specifications for system coexistence be as simple as possible. Although the CDCF signal is based on the OFDM system on the assumption that it has been adopted by a majority of today's high-speed modems, the modems themselves cannot exchange detailed information. The most they can do is to notify one another of the presence of a master station that performs band reservation.